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Scientists have moved a step closer to growing live human organs for transplant by showing that human embryonic stem cells can be encouraged to grow on scaffolds into three-dimensional structures, according to a new study.

The researchers also report they transplanted these structures into special laboratory mice and found they continued to express human proteins and began to integrate with their hosts' blood supply.

"I think it's a long way from any organ but it's a significant step along the way for, say, a pancreas or a liver maybe 10 years down the track," an Australian authority on stem cell technology, Professor Alan Trounson of Melbourne's Monash University told ABC Science Online.

The new research builds on the rapidly advancing field of tissue engineering, which combines materials science and developmental biology. Thus far it has been well established that stem cells cultured on a flat surface, such as a glass dish, can be induced to become various cell types. But the drawback is that cell proliferation can't be well controlled nor produce more complex structures.

Previous research on scaffolds made of biodegradable polymers has already used ordinary adult cells taken from humans and other species and shown that they can be used to grow more complex organ-like structures. ABC Science Online reported earlier this year, for example, that U.S. scientists had successfully grown a rabbit's penis in a laboratory and had re-grown and restored the function of severed penile nerves in rats. Tissue-engineered bladders have also been grown and implanted successfully in dogs and whole human bladders have been designed.

The difference with the new research by Levenberg and team is that it used human embryonic stem cells: "Stem cells aren't necessary for tissue engineering," Trounson says. "But the importance of an embryonic stem cell population is that it might form all the cell types in an organ - but a lot more work needs to done before that could happen."

The researchers strategically placed different growth factors and physical cues within porous scaffolds to influence how and where cells would differentiate and grow: "The scaffold provides physical cues for cell orientation and spreading, and pores provide space for remodelling of tissue structures," they say.

They then seeded the scaffolds with human embryonic stem cells, which multiplied and lined the scaffolds' inner and outer surfaces. Within two weeks, the cells had begun to organise themselves in three-dimensional structures resembling primitive types of different tissues - depending on which growth factors had been added to the scaffolds.

The researchers suggest that growing human tissues on such scaffolds - which break down and dissolve harmlessly after transplantation - holds promise for over-coming a shortage of donor organs in transplant programs, and could be useful for studying early organ development.